Liquid crystal display devices (LCD) have been produced by filling a liquid crystal composition in a sealed cell formed between two substrates provided with transparent electrodes. LCD have been practically used in various modes such as twist nematic (TN) mode, super twist nematic (STN) mode, and thin-film transistor (TFT) mode since their electric power consumption is small compared with CRT (cathode ray tube display) and since downsizing and weight-lightening are possible. Among the modes, active matrix LCD (AM-LCD) such as TFT have been watched as a prospective winner of flat display accompanied with the progress in actualization of colored display and fine picture image.
Following characteristics are required of the liquid crystal compositions for AM-LCD:
1) Proper optical anisotropy (An) can be produced depending on cell thickness. PA1 2) Voltage holding ratio (VHR) is high to maintain a high contrast of LCD. PA1 3) Electrooptical response speed is high to cope with dynamic image. PA1 4) Range of nematic liquid crystal phase is wide depending on application environment (wide range). PA1 5) Proper threshold voltage (V.sub.th) can be obtained depending on driving circuit. PA1 3-HHB(F,F)-F 15.0% PA1 2-HHB(F)-F 28.4% PA1 3-HHB(F)-F 28.3% PA1 5-HHB(F)-F 28.3% PA1 T.sub.NI =110.7.degree. C. PA1 T.sub.SN &lt;0.degree. C. PA1 .eta.20=25.0 mPa.multidot.s PA1 .DELTA.n=0.077 PA1 V.sub.th =2.32 V PA1 VHR=98.8% PA1 7-HB(F,F)-F 10.0% PA1 2-HHB(F,F)-F 25.0% PA1 3-HHB (F,F)-F 35.0% PA1 5-HHB(F,F)-F 18.0% PA1 7-HB (F)-F 12.0% PA1 T.sub.NI =42.9.degree. C. PA1 T.sub.SN &lt;0.degree. C. PA1 .eta.20=22.2 mPa.multidot.s PA1 .DELTA.n=0.059 PA1 V.sub.th =1.07 V PA1 VHR=98.7% PA1 2-HHB(F,F)-F 26.0% PA1 3-HHB(F,F)-F 26.0% PA1 5-HHB(F,F)-F 26.0% PA1 7-HB(F)-F 12.0% PA1 5-H2B(F)-F 10.0% PA1 T.sub.NI =46.0.degree. C. PA1 T.sub.SN &lt;0.degree. C. PA1 .eta.20=21.6 mPa.multidot.s PA1 .DELTA.n=0.058 PA1 V.sub.th =1.17 V PA1 VHR=98.5%
AM-LCD have adopted for driving a TN display mode in which the molecular orientation of a liquid crystal composition filled between an upper and a lower substrate is twisted by 90.degree.. In this TN display mode, coloring of liquid crystal cells due to interference caused when voltage is not applied is a problem. In order to avoid the coloring and to obtain an optimum contrast, the product (.DELTA.n.multidot.d) of .DELTA.n and cell thickness d (.mu.m) must be established to a certain value, for example, to 0.5 .mu.m. Since such restriction exists, a main current of .DELTA.n of liquid crystal compositions for TFT currently used in practice is generally about 0.07 to about 0.11 and particularly 0.08 to 0.10 for 1st. Min. system.
In recent years, demand for developing high response speed LCD is strong for the purpose of coping with dynamic image. Since response speed (.tau.) is proportional to the viscosity (.eta.) of liquid crystal material, it is necessary to explore liquid crystal compositions of a low viscosity to achieve the high response speed.
With the development of portable LCD as a momentum, exploration for LCD intended for outdoor use has come to be investigated. In order to withstand outdoor use, liquid crystal compositions are considered to be necessary to exhibit a nematic phase over a range beyond the temperature range of application environment.
With respect to portable LCD, downsizing is desired to lessen their weight, whereas there are many restrictions in the aspect of driving electric power.
In order to cope with such situation, liquid crystal materials of small electric power consumption and low V.sub.th are desired. From such viewpoint, liquid crystal compositions having a nematic-isotropic phase transition temperature (clearing point T.sub.NI) of 60.degree. C. or higher and smectic-nematic phase transition temperature (T.sub.SN) of lower than -20.degree. C. have become a main stream of liquid crystal compositions for TFT currently used in practice.
In order to respond to such demand, various types of liquid crystalline compounds and liquid crystal compositions containing the compounds have been developed. For instance, Laid-open Japanese Patent Publication No. Hei 2-233626 has disclosed in its Application Example 2 a composition comprising 15% by weight of a trifluoro compound having a comparatively large dielectric anisotropy (.DELTA..epsilon.) and 85% by weight of a difluoro compound. However, this composition has defects that V.sub.th is high, the miscibility of the components in the composition is deteriorated particularly at low temperatures, and nematic phase range is narrow.
WO 94/03558 has disclosed compositions comprising a trifluoro compound and a difluoro compound. However, the compositions disclosed in its Examples 1 and 2 have such a low clearing point as lower than 50.degree. C. and have a .DELTA.n of lower than 0.06, and thus are short of practical utility. Compositions disclosed in its Example 4 and after have a defect of having a high V.sub.th.
Liquid crystal compositions are diligently being studied depending on various purposes as discussed above, they are still not sufficient, and it is a present situation that new improvements are all the time demanded.